The invention concerns a process for curing a UV curable printing ink on a printed material, wherein the printing ink is irradiated with UV light from a UV radiation source. The invention also concerns an associated device for irradiating the printing ink with UV light.
UV curing printing inks contain low amounts of solvent or no solvent, cure when irradiated, and have recently become increasingly important. This is due to the high energy of UV radiation which is particularly advantageous in high speed printing of printed materials, in particular for flat bed printing and letter press printing. They also have practical advantages from a technical applications point of view compared to solvent-containing ink e.g. with regard to their working lifetime, solvent related environmental pollution, and waste disposal.
UV curing printing inks have a UV curable fixing agent system comprising a polymerizing fixing agent or mixture of fixing agents and one or more associated photo-initiators. The polymerization or cross-linking can be triggered by UV irradiation to cure the ink. One differentiates between radical-induced and cationic polymerization. Conventional radical-induced polymerizing fixing agents are based on acrylates, whereas the cationic polymerizing ones are characterized by acid release during UV irradiation. The invention concerns the general curing of UV curing printing inks independent of the particular fixing agent system.
Conventional applications for UV curing printing inks are e.g.: sheet-fed offset printing (e.g. packaging), continuous offset printing (e.g. direct mail advertising), dry offset printing (indirect letterpress printing, e.g. cups and tubes), label printing (letterpress and flexographic printing), flexographic printing (e.g. packaging) and silk screen printing (e.g. technical articles). UV curing, also often referred to as UV drying, has the advantage that the printing inks are solvent-free or of low solvent content and cure rapidly on the printed material under UV irradiation so that the printed material can be promptly further processed or packaged. The invention concerns curing of the printed ink and is therefore independent of the particular printing process used for introducing the printing ink onto the printed material.
Substantial technical requirements are required for the industrial radiation curing of printing ink. Prior art has required very high output power for the UV radiation sources in order to satisfy demands for ever increasing production speeds of 100 to 400 m/min and higher In multi-color printing, the separation between printing devices must be kept small to guarantee the precise matching of sequential colors without excessive complication and expense. The maximum separations in combination with the high printing speeds lead to extremely short times within which the ink must be sufficiently cured to prevent smearing during subsequent handling. Practical separations between printing devices assume values of circa 0.3 to 1.0 m corresponding to production times between printing stations of about 0.1 sec.
When one considers these stringent requirements it becomes clear that the UV intensity of the radiation source is very important. In order to achieve this, mercury vapor high pressure and medium pressure lamps have been nearly exclusively used as UV radiation sources in practical industrial applications up to this point in time. These lamps facilitate a particularly high UV intensity. DE-3902643 C2 and DE 4301718 A1 provide examples therefor.
The arc lengths of the conventional lamps vary between 10 cm and 220 cm and the specific electric power lies in the range between 30 to 250 watts per centimeter of arc length. The UV light power assumes values of approximately 20 watts per centimeter of arc length. Due to the need for U transparency, the tubular lamp material is quartz and the lamps are operated with a gas pressure of 1 to 2 atm. In certain cases, lasers, in particular excimer lasers, are also used to produce the UV radiation.
The above mentioned conventional UV radiation sources have the advantage of being able to produce a very high UV intensity on the surface of the printed material to effect very short curing times in the range of tenths of seconds. Excimer lasers have the disadvantage cf being complicated and expensive. For this reason, medium and high pressure gas discharge lamps are more widely used. They have, however, the disadvantage that their efficiency of UV light production in the relevant spectral region is only 20%, so that 80% of the introduced energy is dissipative power and must be removed by cooling.
Due to the high power consumption and high dissipative power of the lamps, their surface temperatures are in the range of 800 to 900.degree. C. which necessitates special measures for cooling their surroundings. Since the lamps cannot be immediately restarted after having been switched-off, one must also provide means for preventing the printing ink introduced onto the printed material or the printed material itself from burning when the printing machine is in paused operation. Heat protection glass, which is sometimes cooled, and pivoting reflectors are therefore provided. The power consumption portion of the drying device used in a conventional printing machine having an overall power consumption of 100 kW, assumes values in excess of 50 kW and typically 80 kW.
Conventional use of medium pressure and high pressure lamps is therefore very complicated and expensive and is associated with high power consumption. The associated disadvantages have, however, been accepted in the art of radiation curing printing technology, since one has assumed up to this point in time that very high intensity UV lamps having high UV radiation power were necessary to achieve shorter curing times.
The publication Industrie-Lackier-Betrieb [Industrial Coating and Painting], 1969, pages 85-91 proposes the use of so-called actinic or super-actinic fluorescent lamps to reduce thermal loads when curing UV curable coatings. These are special low pressure lamps having a fluorescent coating which shifts the intensity maximum towards the red to achieve a spectrum having high fractions in the UV-A region. The high UV-A fractions have been considered necessary by those of average skill in the art in order to achieve rapid reaction times. Those skilled in the art of curing pigmented systems such as printing inks were of the same opinion. JP 59189340 A2 (Derwent reference No. 84-303796/49) proposes a compound for use as printing ink which can be cured by a plurality of different UV radiation sources, including high pressure, medium pressure, and low pressure mercury lamps. The applications described in this publication suggest that the lamps primarily emit in the UV-A or visible spectral region and that relatively long irradiation times, not compatible with rapid industrial production processes, were required.